"I was freaking out. It was a six-hour surgery that they had to do on me, and the normal kidney process is actually two hours," she said. "Apparently one of the arteries had flipped. I'm very thankful that there are people who know what to do in that situation."

Raisa, 29, and Gomez were roommates at the time. Gomez said she couldn't believe that Raisa would be willing to donate a kidney, let alone be a match.

"She lived with me in this interesting time where my kidneys were just done. That was it and I didn't want to ask a single person in my life," she said. "The thought of asking somebody to do that was really difficult for me. And she volunteered and did it…The fact that she was a match, I mean, that's unbelievable."

The singer said the transplant made her arthritis go away and that there is a 3% to 5% chance that her lupus returns.